1. Technical Field
The present invention relates to a method and apparatus for determining vehicle speed and more particularly to a method and apparatus for calibrating vehicle speedometer signals.
2. Background to the Invention
Poor wheel alignment and tire under inflation are factors contributing to premature tire wear and poor vehicle handling. A system which alerts a vehicle operator of low tire pressure and misalignment would help control one of the factors contributing to the accelerated wear of tires. Unfortunately, the direct provision of pressure sensors in wheels to monitor tire pressure has proven unreliable and expensive. Pressure sensors mounted in wheels are subjected to a hostile environment of high temperatures, road shock and high rotational velocities. The data transmission linkage from the wheel to the vehicle is necessarily indirect and adds expense to the system, and it requires an independent power supply, such as a battery, which must periodically be removed from the tire for replacement.
Tire pressure directly relates to the rolling radius of a tire, which shrinks with reduced pressure. As the tire""s rolling radius is compressed, side wall flex increases causing the tire""s temperature to increase. Increased operating temperature of a tire promotes degradition of the synthetic rubber of the tire and results in excessive tire wear. As an alternative to measuring tire pressure, a wheel""s rotational speed can also be measured as an indirect indication of tire under inflation. The increasingly widespread use of antilock braking systems (ABS) on cars and trucks affords a convenient source of wheel rotational speed information which can be used to implement pressure monitoring methods involving wheel rotational speed monitoring.
U.S. Pat. No. 6,064,936 to Nakajima describes a system utilizing an ABS to provide inputs for tire pressure monitoring system. While increases in one wheel""s velocity suggest low inflation of the tire mounted on that wheel, other circumstances may exist which be the cause of a relative increase in wheel angular velocity. The Liu, et al. reference, U.S. Pat. No. 5,760,682 develops calibration coefficients to compensate for factors such as wheel slippage, rough road fluctuations, vehicle cornering, and uphill and downhill travel. Desirable though is a system which can identify different conditions affecting wheel rotational velocity so that the proper corrective steps may be taken.
Numerous patents relate specifically to the application of indirect tire pressure measurement using ABS wheel speed sensors on automobiles. With trucks additional issues are raised due to the much higher sidewall stiffness of tires compared to automobile tires (and consequent relatively lower sensitivity of rolling radius to tire pressure) and the differences in wheel layout used for trucks, for example 4xc3x976 (a front steering axle with two tires with a rear driven axle carrying four tires) and 6xc3x9710 (a front steering axle with two rear driven axles, each with four tires) wheel arrangements. Truck suspensions are designed for maximum load conditions and loading varies much more than for passenger autos. Vehicle speed for trucks is commonly measured by a tachometer. The tach is attached to track the rotational velocity of a drive shaft installed between the vehicle""s transmission and a rear end differential. As a result measured velocity is proportional to the average rotational velocity of the driven wheels. ABS wheel sensors provide an alternative source of data with which to generate vehicle speed, among values for other operating variables.
The objects, features, and advantages of the invention will be apparent from the written description that follows. The invention provides an apparatus and a method for detecting compromised vehicle wheel and tire operating characteristics. Among the conditions indication of which can be detected are out of round new tires, under pressure tires, poor front wheel alignment and either a longitudinally or latitudinally off center load. The invention provides that an instantaneous rotational velocity signal for each wheel is generated, typically using an existing antilock braking system (ABS). Wheel speed is compared against an instantaneous indication of vehicle speed, which is usually, but not necessarily, generated from the rotational speed of the drive shaft take-off from the vehicle""s transmission.
Operation of the method and system requires calibrating newly mounted tires. As part of the calibration process it is determined if each newly installed tire is within tolerances for the tire and if the front tires appear to be correctly aligned. Calibration is done to generate a scalar relating wheel rotational velocity for each wheel to straight line vehicle speed. Once scalars have been obtained for each wheel they are stored, and then adjusted for estimated wear of the tires as a function of distance traveled. To obtain these numbers, the distance traveled on the tires currently mounted on the wheels and distance traveled since a current trip began are tracked. Vehicle speed can be calibrated utilizing the global positioning system, which involves installation of the appropriate sensor on the vehicle.
Operational monitoring of the wheels includes initializing the trip odometer, collecting wheel speeds, and sampling vehicle speed. Tire radius and tolerance data are generated as a function of estimated wear and tire temperature. Tire temperature is in turn a function of ambient temperature and trip distance. Rotational velocity of the wheels are estimated as functions of vehicle speed and the updated tire radius data. Rotational velocity for each wheel is subtracted from estimated velocity and the difference is compared to the allowed, updated tolerances. A possible pressurization error is indicated if one wheel departs outside of the updated tolerances from estimated rotational velocity.
Additional effects, features and advantages will be apparent in the written description that follows.